1. Technical Field
This invention relates to integrated circuit (IC) fabrication and, in particular, metal fuses having a low height in the end die. One technique to achieve this is to reduce the thickness of the metal fuse during die fabrication.
2. Description of the Related Art
Laser trimming is a technique employed during die fabrication to achieve specific tasks. For example, laser trimming may be used to result in accurate resistors, to re-route circuits, to repair defective dies, and to optimize test distribution of electrical wafer sorting. After electrical wafer testing and sorting, laser trimming is typically used to burn metal lines of a die that are fuse lines to repair defective circuits or dies.
Fuse lines are used to connect redundant circuits or disconnect defective circuit components. The material of fuse lines in ICs may be either poly silicon or metal. Poly silicon fuse lines are usually used in layers located far deep in the finished wafer. This renders fuse window processes more difficult. Thus, for multi-metal-layer ICs, metal fuse lines are preferred. Typically, metal lines formed from the second to last metal layer are chosen to be used as the fuse lines. In order to save production cost and to achieve process simplicity, fuse lines and signal and interconnect lines are often formed from the same metal layer. The metal layer thickness and the minimum critical dimension are typically defined by the fabrication technology. For example, in certain technologies that process silicon wafers with bipolar-CMOS, the metal layer from which metal fuse lines are formed is the metal 2 layer with a thickness that varies from 0.5 μm to 1.1 μm.
It is well known that narrow width and low aspect ratio of fuse lines are desired from the perspective of laser trimming efficiency. A metal fuse line for laser trimming is preferred to have a thickness of no more than 0.5 μm so the fuse can more easily be blown. On the other hand, signal lines and interconnect lines have a lower resistance if they are made thicker and wider. A signal line with a thickness over 1 μm is preferred. Metal fuse lines are formed from the same metal layer as interconnect lines, and they may have a thickness of greater than 0.5 μm. Consequently, higher laser pulse energy is required to burn and remove thicker metal fuse lines. Using higher energy, however, raises the risk of causing damage to lower layer/layers, the substrate, and/or to neighboring fuse or circuit lines. Other undesirable problems caused by the use of higher energy in laser trimming include: fluctuation of trimming efficiency, metal residual, and inter-layer dielectric cracking.
There is thus a tradeoff of having lower resistance interconnect lines or easy to blow fuse lines if both are made in the same layer.